1. Field of the Invention
The present invention relates in general to the field of signal processing, and, more specifically, to apparatuses and methods for providing an adjustable constant current source which operates in both Discontinuous Conduction Mode (DCM) and Continuous Conduction Mode (CCM).
2. Description of the Related Art
Switch-mode systems are well known in the art. FIG. 1 shows an exemplary switch-mode system 100. Switch-mode system 100 includes a switch-mode converter power stage 102, a converter current controller 104, and a load 106. Power 101 is fed into switch-mode converter power stage 102. Switch-mode system 100 utilizes switch-mode converter power stage 102 to convert alternating current (AC) voltages (such as line/mains voltages) to direct current (DC) voltages or DC-to-DC wherein the input current is proportional to the input voltage. Converter current-controller 104 controls the current of switch-mode converter power-stage 102, and switch-mode-converter power stage 102 accordingly drives load 106. An exemplary switch-mode converter power stage 102 may be a buck converter or a boost converter. For switch-mode converter power stage 102, two modes of switching stage operation exist: Discontinuous Conduction Mode (“DCM”) and Continuous Conduction Mode (“CCM”).
In CCM, the switch of switch-mode-converter power stage 102 switches “ON” by converter current controller 104 when the inductor current iL of converter current controller 104 is non-zero, and the current in the energy transfer inductor of converter current controller 104 never reaches zero during the switching cycle. FIG. 2A shows exemplary target current itarget (for inductor current iL) having a value of three (3) amp. FIG. 2A further shows inductor current iL has a period of ten (10) microsecond and is always non-zero in value. In CCM, the current swing is less than in DCM, which results in lower I2R power losses and lower ripple current for inductor current iL which results in lower inductor core losses. The lower voltage swing also reduces Electro Magnetic Interference (EMI), and a smaller input filter can then be used. Since the switch of switch-mode converter power stage 102 is turned “OFF” when the inductor current iL is not equal to zero, the diode of switch-mode converter power stage 102 needs to be very fast in terms of reverse recovery in order to minimize losses. In DCM, a switch of switch-mode converter power stage 102 is turned on (e.g., “ON”) by converter current controller 104 when the inductor current iL of the inductor of switch-mode converter power stage 102 equals zero. FIG. 2B shows exemplary target current itarget having a value of 0.8 amp. FIG. 2B further shows inductor current iL has a period of ten (10) microsecond and does fall to the zero value at a portion of each period.
As an example, for a switch-mode system 100 used to drive a Light Emitting Diode (LED) lighting system, it is important to have an accurate constant current output that is adjustable across a wide dynamic range, such as a range of one hundred-to-one (100:1). Such an accurate adjustable constant output requires a switch-mode converter power stage 102 that can smoothly transition between CCM and DCM and provide a controlled output in both CCM and DCM. For high target current values itarget that are set for inductor current iL, the current ripple needs to be minimized. Such low current ripple generally calls for operation of the switch-mode converter power stage 102 to be in CCM. Unless the inductor of switch-mode converter power stage 102 is greatly oversized, switch-mode converter power stage 102 will operate in DCM at a lower average current iaverage for inductor current iL. If the current of switch-mode converter power stage 102 in CCM mode is controlled in such a way that the time above target current itarget and the time below that target current itarget are equal, then the average current iaverage is equal to the target current itarget, assuming linear (non-saturating) operation of the inductor. CCM operation of switch-mode converter power stage 102 generally does not require knowing the inductor value L (or scaled: inductor constant F*L product wherein F may be an on-time period divided by the count value of the on-time period) nor the input/output ratio D in order to provide such accurate constant average current output.
However, providing a smooth transition between CCM and DCM for a converter power stage 102 has its challenges. For example, in DCM, the inductor value L (or scaled inductor F*L product) and input/output ratio D need to be known since they directly affect the current calibration. Any inaccuracy will cause a differential non-linearity (“DNL”) error in the effective current Digital-to-Analog Converter (“DAC”). Thus, in order to provide at least a smoother transition between CCM and DCM, there are needs and desires to discover, observe, and/or derive the inductor value L (or scaled inductor constant F*L product) and the input/output ratio D of a switch-mode converter system, particularly when the switch-mode converter system operates in DCM. The attaining of such measurements allows the on-time or the total operating period for the switch of the switch-mode converter 102 to be controlled so that an accurate adjustable constant average current output for the inductor current iL is provided.